Biology Reference
In-Depth Information
3.5 Composition of Insect DNA
Insect DNA base ratios are lower than those found in vertebrates, with G
+
C com-
prising from 32 to 42% of the DNA, compared with 45% for vertebrates (
Berry
1985
). If base composition were random, 50% of the DNA would be G
+
C.
DNA in eukaryotes can occur in different configurations. Most genomic DNA
exists in the B-helix form, but other configurations are known, including
triplex
DNA
. In triplex DNA, the usual A-T and C-G base pairs of duplex DNA are pres-
ent, but in addition a pyrimidine strand is bound in the major groove of the
helix. DNA sequences that potentially can form triplex DNA structures seem to
be common, are dispersed at multiple sites throughout the genome, and com-
prise up to 1% of the genome. Triplex DNA was identified in polytene chromo-
somes of
Chironomus tentans
and
D. melanogaster
, where it was found in the
euchromatic bands (
Burkholder et al. 1991
).
3.6 Chromosomes are DNA Plus Proteins
Eukaryotes must organize and package their DNA in a sufficiently condensed
form that it can it into a very small space in the nucleus during mitosis and
meiosis. Yet, this packaging must be compatible with the ability to separate
the DNA strands and unwind the DNA helix during DNA replication and tran-
scription during interphase. Furthermore, the packaging must occur rapidly.
Precise and rapid replication of DNA is required in many tissues (
Koshland and
Strunnikov 1996, Nicklas 1997
).
Eukaryotic genes are located on linear DNA molecules, with each chromo-
some containing a single long DNA molecule. In addition, each chromosome
contains an approximately equal amount of proteins with different functions.
The proteins include DNA and RNA polymerases and regulatory proteins associ-
ated with the DNA. At least five
histones
are associated with the DNA in struc-
tures called
nucleosomes
(
Figure 3.1
). Histone proteins (called H1, H2A, H2B,
H3, and H4) contain
≈
100-200 amino acids, of which 20-30% are arginine and
lysine. As a result, the histones have a positive charge that helps histones bind to
DNA. Genes that code for the major histones are very highly conserved among
eukaryotic species, indicating that they are nearly unchanged over billions of
years. Histones are crucial to maintaining chromosome structure, and they may
be crucial to the effective function of DNA as the genetic code (
Jenuwein and
Allis 2001
).
Core DNA (the DNA in a nucleosome) is connected by
linker DNA
to the
next nucleosome (
Figures 3.1, 3.2B
). The
core DNA
is protected from digestion
